Hopkins Scientists Find Key to Spring Mating
in Brain Chemistry

Johns Hopkins University scientists have discovered a key
brain mechanism that may help to explain why some animals are
much more likely to mate in spring than at other times of the
year.

The researchers, studying a small mouse-like mammal called a
prairie vole, have discovered that in the spring, compared with
winter, the animal's brain produces much more of a protein that
indicates that nerve cells essential for mating have been
activated.

The findings were detailed in a scientific paper published
April 17 in the journal Brain Research.

Scientists already knew that voles produce the protein when
they are exposed to a specific chemical scent called a pheromone,
which is contained in the male's urine and other secretions.
Female voles show no interest in mating with males unless they
are exposed to the appropriate chemical signal from the males'
urine. The chemical stimulus is converted in the brain into a
signal that triggers the release of hormones in the female,
leading to mating behavior, said Randy Nelson, an associate
professor in the Johns Hopkins Department of Psychology.

"It sounds rather unpalatable but remember, most non-human
animals go around sniffing each other," said Gregory Ball,
another associate professor of psychology who is involved in the
research. "We are kind of a strange mammal in that we don't use
olfaction in the way that most of our relatives do."

Hopkins researchers wanted to test their hypothesis that the
animals might actually be more sensitive in springtime to the
chemical signals behind reproduction.

"So we presented the female with urine at different times of
the year, and then we looked at the effects on cells in the
brain," Dr. Ball said.

The hypothesis turned out to be correct: The female voles'
brains produced more of the protein, called fos, in lab-simulated
springtime conditions than during simulated winter.

The absence in winter of springtime levels of the protein,
even after exposure to the urine, is critical. It indicates that
the urine smell in winter did not trigger the cascade of events
in the nervous and endocrine systems that leads to mating.

Scientists believe that an explanation for this seasonal
difference in protein production, and thus for the lack of
wintertime sexual motivation, may lie in changes in brain
physiology as the number of daylight hours increases and
decreases. Additional research is testing that idea.

In humans, although women have major changes in hormone
production during ovulation, they do not experience estrus, or
cyclic willingness to mate. Human sexual behavior is not so
profoundly regulated by hormones.

"That is a very important thing that affects our social
system, that we can engage in mating at any time in the cycle,"
Dr. Ball noted.

Also working on the research was former doctoral student
Christopher Moffatt, who has since received his doctorate in
psychology and is currently a research assistant professor at the
University of Massachusetts at Amherst. The three researchers
specialize in behavioral neuroendocrinology, which deals with how
the brain responds to hormones.

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